A method of etching a multilayer film having an oxide layer is known as a conventional plasma processing method. See, for example, Japanese Patent Laid-Open Publication No. 2005-26659. The method disclosed in Japanese Patent Laid-Open Publication No. 2005-26659 is a manufacturing process of a NAND type flash memory and uses a tungsten thin film as a hard mask for etching an oxide layer. The thickness of the tungsten thin film is in the range of about 500 Å to 1000 Å such that the tungsten film may sufficiently endure as an etching barrier during the subsequent etching of the oxide layer is etched.
Also, Japanese Patent Laid-Open Publication No. 2009-170661 discloses an example of a NAND type flash memory. This NAND type flash memory has a three-dimensional structure in which memory cells are stacked in the vertical direction in relation to the surface of a semiconductor substrate. This memory has multi-layered wiring layers in which a plurality of conductive films and a plurality of insulation films are alternately stacked and contacts configured to supply potentials of word lines. In order to form the contacts, a plurality of end portions of the multi-layered wiring layers are worked in a step shape. That is, the lengths of the multi-layered wiring layers are set in such a manner that the lowermost wiring layer has the longest length and the uppermost wiring layer has the shortest length and thus, the lengths of the multi-layered wiring layers get shorter from the lowermost wiring layer towards the uppermost wiring layer.